Computer Networking Quality-of-Service (QoS) Dr Sandra I. Woolley

2 Quality-of-Service (QoS)  IP has proven to be extremely powerful because of it was designed to provide internetworking capabilities over a variety of network technologies.  However, traditional IP networks can only provide best-effort service and so do not readily support applications that require real-time response.  ATM (Asynchronous Transfer Mode) is an example of a network architecture that was designed at the outset to provide end-to-end Quality- of-Service (QoS) to support a wide range of services such as voice, data and video. –ATM was developed in the mid-1980s as a high-speed multi-service network technology. It is a connection-oriented, packet-switching, QoS supporting technique that uses small fixed length (53 byte) packets called cells. –ATM disadvantages included the inefficient header (5 bytes) and the need to implement segmentation and reassembly at high speeds. ATM also does not have the scalability and cannot achieve the ubiquity of IP.  Interest in supporting Quality-of-Service in IP networks produced the Integrated Services (IntServ) and Differentiated Services (DiffServ) models.

3 IntServ and DiffServ Summary  IntServ and DiffServ: Two IETF models to support different classes of services.  Integrated Service (IntServ) –Establish path –Network resource is reserved based on the QoS requirements of each traffic flow –Hops along the path keep the reservation information  Differentiated Service (DiffServ) –Request Service and set up Service Level –A Service Level Agreement (SLA) is made with the Internet Service Provider (ISP). –Packets are marked based on SLA –Traffic flows are aggregated based on requested class of service in DiffServ domain

4 IntServ and RSVP  IntServ uses a protocol called RSVP (ReSerVation Protocol) to reserve route resources along the flow path.  The reservations are soft state (automatically time-out if not refreshed.)  IntServ services are called guaranteed and controlled-load.  Guaranteed services were designed for real-time delivery strictly within a certain time limit whereas the controlled-load services were designed for traffic that could tolerate some delay but would be sensitive to traffic overload conditions.

5 IntServ Complexity and Scalability  The IntServ model was a first step toward providing QoS in the Internet. –However, the IntServ model requires routers to keep flow-specific states for each flow. The amount of state information increases proportionally with the number of flows so routers would need large memory and high processing power. –IntServ also requires routers to implement RSVP which requires admission control and packet classifying and, in particular, a per- flow packet scheduling algorithm.  Because of scalability and complexity issues IETF developed another model called Differentiated Services (DiffServ or DS).  You can read the original IETF RFCs – (IntServ) – (DiffServ - see also 2474)

6 DiffServ  Unlike IntServ which requires an application to make a resource reservation for each flow, the DiffServ model aggregates the entire customer's requirement for QoS. A customer or organization wishing to receive differentiated services must first have a service level agreement (SLA) with an ISP. This contract could be static or dynamic.  When the customer wishes to receive different levels for different packets, they need to mark packets by assigning specific values in the type-of-service (TOS) field (renamed to the DS field). These values determine the per-hop-behaviours (PHB) at each router in the DiffServ domain.

7 DiffServ  One approach for dynamically controlling DiffServ bandwidth involves the use of a Bandwidth Broker.  Multi-Protocol Label Switching (MPLS) is used to label traffic.  Services include Expedited Forwarding and Assured Forwarding. –Expedited forwarding provides end-to-end low-loss, low- latency, low-jitter, assured bandwidth. –Assured forwarding delivers traffic with high probability of successful delivery. It is not intended for low-latency or low- jitter applications. –Four independent types of Assured Forwarding have been defined, each can have one of three drop precedence values. These define the relative importance of packets in congested routers.

8 Summary QoS Quality-of-Service A short introduction to QoS networking Integrated Service (IntServ) Differentiated Service (DiffServ)

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